Frontend/SDL - Provide proper default for UDP input

When the default file is read in, the settings default value is only used
when the key is missing. As it was, the key existed, but the value was empty string
causing it to accept that as a value to pass into the core

src/yuzu_cmd/default_ini.h

@@ -84,7 +84,7 @@ touch_device=
 # from any cemuhook compatible motion program.
 
 # IPv4 address of the udp input server (Default "127.0.0.1")
-udp_input_address=
+udp_input_address=127.0.0.1
 
 # Port of the udp input server. (Default 26760)
 udp_input_port=